In the female rat there exist two prolactin biorhythms that are involved in the physiology of the reproductive cycle. One rhythm (or surge) occurs on the afternoon of proestrus, is estrogen induced, and is luteolytic to the previous set of corpora lutea. A second rhythm is induced as a result of copulation and occurs in the early morning hours and is luteotropic, i.e., stimulates progesterone secretion and maintains pregnancy. The purpose of this proposal is to determine what regions in the central nervous system are involved in the generation of the surges and where are the steroids acting in the central nervous system to amplify the surges. In previous studies using a free-running, aortic-catherized, ovariectomized rat treated with a long acting estrogen, polyestradiol phosphate, daily afternoon surges were induced. Lesions placed in the corticomedial amygdala attenuated the surge but did not eliminate it. The neural generator of this surge is unknown but evidence suggests that the suprachiasmatic nuclei of the hypothalamus may be the source. Our hypothesis then is that the suprachiasmatic nucleus induces a daily afternoon prolactin surge in the female rat and when estrogen is present the corticomedial amygdala amplifies the surge to a level of physiologic significance to the estrous cycle of the rat. Studies will be undertaken to verify this hypothesis by discrete radiofrequency lesion of the suprachaismatic nuclei and by local estrogen implants into the corticomedial amygdala suprachiasmatic nuclei, anterior pituitary and other regions of the brain using the ovariectomized rat with an indwelling aortic catheter. An animal model for the nocturnal biorhythm will be developed using an ovariectomized, hysterectomized rat with progesterone silastic capsule implants and an indwelling aortic catheter. Areas in the brain will be lesioned to block the progesterone-induced PRL surge and when located, brain progesterone implants will be made to mimic systemic exogenous progesterone administration.